and the Mode of its Communication. 7 1 



cold, placed at the same distance ; and that a bullet at 

 the temperature of freezing mercury could not affect us 

 much more sensibly, by its frigorific rays, than an equal 

 bullet at the temperature of boiling water would do 

 by its calorific rays ; but at these comparatively small 

 intervals of temperature, the radiations of bodies are 

 hardly sensible, and could never have been perceived, 

 much less compared and estimated, without the assist- 

 ance of instruments much more delicate than our or- 

 gans of feeling. Hence we see how it happened that 

 the frigorific radiations of cold bodies remained so long 

 unknown. They were suspected by Bacon ; but their 

 existence was first ascertained by an experiment made 

 at Florence towards the end of the seventeenth century. 

 And it is not a little curious, that the learned academi- 

 cians who made that experiment, and who made it with 

 a direct view to determine the fact in question, were so 

 completely blinded by their prejudices respecting the 

 nature of heat that they did not believe the report of 

 their own eyes ; but, regarding the reflection and con- 

 centration of cold (which they considered as a negative 

 quality) as impossible^ they concluded that the indica- 

 tion of such reflection and concentration which they 

 observed must necessarily have arisen from some error 

 committed in making the experiment. 



Happily for the progress of science, the matter was 

 again taken up, about twenty years ago, by Professor 

 Pictet ; and the interesting fact, which the Florentine 

 academicians would not discover, was put beyond all 

 'doubt. But still, this ingenious and enlightened phi- 

 losopher did not consider the appearances of a reflection 

 of cold, which he observed in his experiments, as being 

 real; nor was he led by them to admit the existence of 



